1. Field of the Invention
This invention relates to a wear resistant surface layer.
2. Description of the Prior Art
In recent years, for higher power generation, the internal combustion engine has been improved for larger compression ratio and higher rate of revolution. However, this leads to exposure of the piston ring, cylinder liner, and other sliding parts to higher working temperatures and formation of thinner lubricant oil film on these parts. As a result, these parts are rubbed more with metal surfaces, getting leading to excessive wear and even scuffing in worst cases. Accordingly, these sliding parts have come to require more wear and scuffing resistance than before.
To endow these sliding parts with the required wear resistance, it has been known to plate the parts with hard chromium. The hard chromium plating thus formed is superior in hardness and has such a desirable wear resistance that both the plating itself and the part surface that is brought in contact thereto wear less. Incapable of retaining oil on its surface, however, the above plating scuffs readily as the plated surface is rubbed.
As a surface layer with improved scuffing resistance, therefore, the spray fused deposit molybdenum surface layer has come to replace the hard chromium layer in some cases in recent years. The surface layer formed by spray fused depositing molybdenum, which is porous enough to satisfactorily retain oil in the surface thereof and has a high melting point, is superbly resistant to scuffing. Being less hard than the hard chromium plating, however, the spray fused deposit molybdenum surface layer is less wear resistant than such plating. Another weak point of the above molybdenum surface layer is susceptibility to oxidation at high temperatures, which means a shorter service life.
To overcome the above weak point of the spray fused deposit molybdenum surface layer, a mixed powder of ferrochrome, molybdenum and nickel-chromium alloy is plasma spray fused deposited to form a surface layer in some cases. Containing molybdenum, however, this type of spray fused deposit surface layer is also susceptible to oxidation at high temperatures, posing a problem of short service life.
On the one hand, types of spray fused deposit surface layers formed by plasma spray fused depositing oxide ceramics, such as titania (TiO.sub.2), alumina-titania (Al.sub.2 O.sub.3 containing 13 or 40 wt. % TiO.sub.2), chromium oxide (Cr.sub.2 O.sub.3), and zironium-yttria (ZrO.sub.2 containing 8 to 12 wt. % of Y.sub.2 O.sub.3), were also tested, which showed weaker binding forces and thereby unsatisfactory adhesion to the underlying surface and susceptibility to chipping.